In conventional rotary valve power steering gears such as described in U.S. Pat. No. 3,022,772, issued Feb. 27, 1962 and assigned to the assignee of this invention, a torsion bar between an input shaft of the gear and an output shaft of the gear simulates road feel when a driver turns a steering wheel connected to the input shaft. A four-way, open-center, rotary valve of the gear includes a spool on the input shaft and a sleeve on the output shaft which cooperate, when the spool rotates relative to the sleeve, to port a first or pressure chamber of a steering assist fluid motor to a pump and a second or exhaust chamber of the fluid motor to a reservoir.
Usually, the rate at which the pressure chamber of the fluid motor expands is the regulated discharge flow rate of the pump. When the steering wheel of the vehicle is turned very rapidly, however, as in an evasive maneuver, mechanical limit stops on the input and output shafts may engage to effect a direct mechanical connection between the input and output shafts. Thereafter, if the high steering wheel turning rate is sustained, the pressure chamber of the fluid motor may be manually over-expanded and the discharge chamber over-compressed. In that circumstance, a partial vacuum may develop in the pressure chamber and over-pressure may be induced the discharge chamber which may increase the manual effort required to sustain the turning rate of the steering wheel. The same result may obtain even without engagement of the limit stops where the steering wheel is turned very rapidly under conditions of low steering resistance such as driving at highway speed or on a slippery road surface where the torsion bar provides sufficient force to move the rack.